Considering how few of them there are, you might think the women appointees of President Reagan would enjoy a degree of immunity from sniping--at least from people who proclaim themselves supporters of the president. But in the case of Loret M. Ruppe, the director of the Peace Corps, exactly the opposite seems to be the case.

She is getting belted around in conservative circles in a fashion that suggests a calculated course of character assassination. The effect is to jeopardize not just her service but, more important, the future of an institution that embodies Reagan's own belief in the power of voluntary service and individual initiative.

The Peace Corps has fallen on hard times. Twenty years after it was launched by John F. Kennedy as a program in which trained American volunteers could share their skills with poor people abroad, the Peace Corps has dwindled from a peak of over 15,000 in 1966 to under 5,000 now.

Yet, the demand from developing countries continues. At the 20th anniversary ceremonies last summer, Prime Minister Edward Seaga of Jamaica said that the volunteers "helping to build a water tank, teaching farmers in the field, bringing literacy to the unschooled, are mightier messengers of international friendship than the treaties and concords which resolve conflicts and hositilities between nations."

Seaga was singled out by Reagan as his first foreign visitor, because the free-enterprising prime minister was the election victor over a pro-Cuban Marxist predecessor. But one of Reagan's favorite conservative journals, Human Events, chose to describe the Peace Corps anniversary as a forum for "virtually every anti-Reagan freak around."

More recently, Human Events returned to the attack, accusing Ruppe of being "less than loyal to the president" in a series of legislative and personnel decisions. That mid-December criticism was echoed and amplified on Jan. l in a nationally syndicated column by Rowland Evans and Robert Novak. It described the Peace Corps as representing, to conservatives, "a repository, within the U.S. government, of Third World animosity."

Ruppe has handed her critics some ammunition. A Carter appointee named William G. Sykes whom she kept on for 10 months as acting director of the Peace Corps' African division gracelessly distributed a parting blast at Reagan's policies to his colleagues. Sykes also announced at his farewell party that he was donating $100 his co- workers had collected for him to the Ohio gubernatorial campaign of former Peace Corps director Richard Celeste, a Democrat. "Shocked" when she learned of the incidents after returning from Africa, Ruppe ordered a complaint filed against Sykes with the Office of Personnel Management. But by then, Human Events and Evans and Novak had their story.

A bit later, she concedes, she apologized to Rep. Dan Lungren (R-Calif.) for her activities on a bill separating the Peace Corps from the ACTION agency, which runs domestic volunteer programs. Lungren was the floor manager for the administration, which opposed the separation. He was told by several GOP members that Ruppe had quietly been lobbying for the independence of her own agency.

She conceded to Lungren--as she did when I interviewed her-- that she had told members of Congress what was obvious to her and her predecessors: that there were serious "administrative difficulties" in the Peace Corps- ACTION merger. A Republican Senate and a Democratic House reached the same conclusion, and voted separation, but Ruppe ended up being accused of disloyalty.

The attack on Ruppe is envenomed by repeated references to her as a pre- convention supporter of George Bush and a social friend of the Bushes in Washington. And the comments also smack to me of sexism, the attitude that she is an "uppity woman" making trouble, instead of staying home in Michigan and helping her husband, a former member of the House, campaign for the GOP Senate nomination.

What is totally lost in the caricature is the serious effort Ruppe and her deputy, former Vietnam POW Everett Alvarez Jr., are making to enlist business and voluntary agency assistance for the Peace Corps and their effort to make it a symbol of Reagan's own idea that self-help and people-to-people assistance are the keys to development.

Ruppe says Reagan himself has been supportive. But, as in so many other instances, there are people around him who seem determined to reject as an alien force anything or anybody who deviates from their own form of conservative orthodoxy. The Peace Corps --an American idea if there ever was one--deserves better than this current campaign of harassment.